Conventionally, retroreflective sheeting capable of reflecting incident light back toward the light source have been well known, and such sheeting is widely used in the above-described fields of application owing to its retroreflectivity. Among others, retroreflective sheeting utilizing the retroreflection principle of prisms, such as cube-corner type retroreflective sheeting, has markedly higher optical retroreflection efficiency than conventional retroreflective sheeting using micro glass beads, and the range of its use is expanding year by year because of its excellent retroreflection performance.
On the basis of their principle of reflection, cube-corner type retroreflective elements exhibit good retroreflectivity, so long as the angle between the optical axis of a prism type reflective element (sometimes referred to simply as "prism element") (i.e., an axis lying at an equal distance from three mutually perpendicular faces constituting the prism element) and incident light (i.e., the angle of incidence) is small. However, cube-corner type retroreflective elements have the disadvantage that, as the angle of incidence increases, their retroreflection efficiency is reduced. Moreover, when rays of light are incident on a prism face at an angle greater than the critical angle satisfying the conditions for total internal reflection which are determined according to the ratio of the refractive index of the transparent medium constituting the retroreflective element to the refractive index of air, most of them do not undergo total reflection at the interfaces of the prism element but pass to the backside of the prism. Thus, they have the additional disadvantage that the range of the angle of incidence which permits retroreflection is limited.
In order to overcome these disadvantages, various attempts have been made to improve the method of making a mold used for the formation of prisms. Now, some typical methods of making a prism mold which have been proposed in the prior art are described below.
(1) Bundled pin method (U.S. Pat. Nos. 1,591,572, 3,922,065 and 2,029,375):
This is a method in which a large number of metallic pins having a prism formed at the tip thereof are bundled to form an array of prisms. This method is characterized in that the design of the prism formed at the tip of each pin may be arbitrarily modified and is suitable for the production of relatively large prisms. However, it is not practicable when the formation of, for example, more than 2,000 microprisms per square centimeter is required as dictated by the primary object of the present invention.
(2) Plate method (U.S. Pat. Nos. 1,591,572, 3,069,721 and 4,073,568):
This is a method for forming a microprism mold of the hexagonal prism type which comprises stacking a plurality of flat sheets having two mutually parallel major surfaces, cutting therein V-shaped grooves in a direction perpendicular to the major surfaces and at a fixed pitch to form a series of successive roof-shaped projections having a vertical angle of about 90.degree., and then shifting the flat sheets so that the vertices of the roof-shaped projections formed on each flat sheet meet the bottoms of the V-shaped grooves formed on an adjacent flat sheet. This method is characterized by a relatively high degree of design freedom, though it is lower than that of the bundled pin method. This method can improve the poor productivity in the fabrication of a prism mold which constitutes a disadvantage of the above-described bundled pin method. However, this method has the disadvantage that, when it is desired to form microprisms, the insufficient strength of flat sheets may cause them to become distorted during the cutting of V-shaped grooves, and has hence been used for the production of relatively large prisms.
(3) Triangular prism method (U.S. Pat. Nos. 3,712,706 and 2,380,447):
This is a method in which V-shaped grooves extending in three different directions are cut in a surface of a flat plate made of a metal or the like to form an array of prisms thereon. This method has frequently been employed for the production of conventional retroreflective sheeting using prism elements. The reasons for this are that it is relatively easy to form microprisms by cutting and that thin retroreflective sheeting may be obtained because it is possible to form an array in which the bases of the formed triangular prisms are arranged in a common plane. However, this method has the disadvantage that the prism shape which can be employed is limited to triangular prisms capable of being formed by V-groove cutting and its degree of design freedom is low.
Next, the properties desired for retroreflective sheeting and the problems involved in cube-corner type retroreflective sheeting using prism elements are described below.
Generally, the basic properties desired for retroreflective sheeting include high brightness properties (i.e., the highness of reflective brightnesses as represented by the reflective brightness of light incident on the sheeting from a direction perpendicular to the surface thereof) and good wide-angle properties. Moreover, wide-angle properties involve the following three considerations.
A first consideration relating to wide-angle properties is observation angle characteristics. Where retroreflective sheeting is used, for example, in various markers such as traffic signs, the position of the viewer is usually different from that of the light source. Accordingly, more intense light must reach the viewer positioned away from the optical axis of the incident light. To this end, it is necessary that the reduction in reflective brightness be small even at large observation angles.
A second consideration relating to wide-angle properties is incident angle characteristics. For example, when an automobile is coming nearer to a traffic sign, the incident angle of light emitted by the headlamps of the automobile to the sign increases gradually, and the intensity of the light reaching the driver being the viewer decreases correspondingly. In order to cause the sign to retain sufficiently high brightness even when the driver comes near to the sign, excellent incident angle characteristics are required.
A third consideration relating to wide-angle properties is rotation angle characteristics. A phenomenon peculiar to prism elements is such that retroreflective brightness varies according to the direction from which light is incident on retroreflective sheeting. Consequently, retroreflective sheeting involves a troublesome problem in that control over the direction of bonding is required in bonding the retroreflective sheeting to signs. Micro glass bead type retroreflective sheeting does not involve this problem because the reflective elements have the form of a body of revolution.
Cube-corner type retroreflective sheeting using prism elements is usually said to be characterized in that the frontal retroreflective brightness thereof is two to three times as high as that of retroreflective sheeting using micro glass beads. The reason for this is said to be that the latter tends to show a reduction in retroreflection efficiency because glass beads generally used in micro glass bead type retroreflectors are optically imperfect for lens elements and tend to produce spherical aberration or because a metallic reflective film formed on reflective surfaces has low reflectivity, whereas prism elements used in the former cube-corner type retroreflective sheeting permit the formation of optical elements having relatively high accuracy. On the other hand, cube-corner type retroreflective sheeting is generally said to have poor wide-angle properties.
Thus, prism elements have higher reflective brightness than glass beads, but are unsatisfactory in wide-angle properties. In order to improve the wide-angle properties of prism elements, further investigation is required with respect to the above-described three types of characteristics. They are more specifically described hereinbelow.
Observation Angle Characteristics:
The beam of light reflected by retroreflective sheeting must have a certain degree of divergence and reach the viewer positioned away from the optical axis of the incident light. To this end, it is necessary to design the retroreflective sheeting so that the reflected light will spread with a small angle of divergence. This can be accomplished by varying the prism edge angles made by adjacent faces of the prism elements very slightly from their theoretical value of 90.degree., by curving the reflective faces of the prism elements slightly, or by utilizing a diffraction effect exerted by the minute prism elements.
On the basis of the relative positions of the headlamps of a transport vehicle (e.g., a large-sized truck) and its driver, the observation angle is usually at most about 3.degree.. Accordingly, the aforesaid angle of divergence should be controlled so as to exceed this maximum observation angle slightly.
Incident Angle Characteristics:
Generally, as the angle of incidence increases, the retroreflection efficiency of retroreflective sheeting is reduced. The reason for this is that, in order to satisfy trihedral reflection requirements based on the retroreflection principle of cube corners, the angle of incidence must be relatively close to 0.degree., i.e., light must be incident on the retroreflective sheeting from a direction substantially perpendicular to the surface thereof. If the angle of incidence becomes greater, the light may fail to reach a second or third reflective prism face and escape out of the prism, resulting in a reduction in retroreflection efficiency. This disadvantage is pronounced especially when triangular prisms are used, and can be mitigated to some extent by using hexagonal prisms. Moreover, if the angle of incidence exceeds a certain limit, the conditions for total internal reflection are not satisfied, so that the incident light passes to the back side of the prism.
In order to overcome the above-described disadvantages, there has generally been employed a method wherein the optical axes of the prism elements, which are conventionally oriented so as to be perpendicular to the surface of the retroreflective sheeting, are slightly tilted in various directions to increase their retroreflection efficiency in the tilting directions.
For example, in the triangular prism method, it has been proposed to vary slightly the angle of intersection of V-shaped grooves which generally intersect with each other at an angle of 60.degree. (U.S. Pat. Nos. 4,588,258 and 4,775,219). Since the optical axes tilted by this method are obtained only in the form of pairs of prisms rotated 180.degree. with respect to one another and facing in opposite directions, an improvement in wide-angle properties can be achieved in the directions of tilting of the optical axes, but no improvement is achieved in other directions. Moreover, no improvement in rotation angle characteristics is achieved.
In order to overcome the above-described disadvantage that the conditions for total internal reflection are not satisfied when the angle of incidence exceeds a certain limit, it has been proposed to coat the reflective faces of prisms with a metal film or the like and thereby cause specular reflection (U.S. Pat. Nos. 3,712,706 and 2,380,447). However, this method has the disadvantage that the resulting sheeting has a dark appearance and the metal film is susceptible to moisture or the like.
Rotation Angle Characteristics:
Rotation angle characteristics pose a serious problem especially in the case of triangular prisms. In order to improve rotation angle characteristics, there is known a method in which the prism array surface is divided into a plurality of zones and the angular orientation of the prisms present in each zone is changed (see U.S. Pat. No. 4,243,618). In this method, the rotation angle of light incident on the prisms differs from zone to zone, and the reflective brightness varies correspondingly. When viewed from a long distance, these reflective brightnesses are leveled to give uniform rotation angle characteristics. However, the zones of the prism array surface can be rather clearly seen from the front side of the retroreflective sheeting, and hence have the disadvantage that the appearance of the sheeting is reduced in attractiveness.
When hexagonal prisms are employed, the shape of the elements resembles a circle more closely. As a result, the failure of trihedral reflection occurs less frequently, resulting in only a slight reduction in rotation angle characteristics. The approximate region of a prism element in which trihedral reflection can be achieved is represented by the inscribed circle thereof, and this occupies about 60% of the projected area of the prism element for triangular prisms and about 90% for hexagonal prisms.
Moreover, in prism molds which can be applied to the fields of application of the present invention and can be used for the production of relatively thin and flexible retroreflective sheeting, it is desirable that the prism elements have a minute size, for example, of 500 .mu.m or less. However, it is difficult to produce such reflective sheeting according to the above-described bundled pin method and plate method. The triangular prism method permits the formation of minute prisms, but it is difficult to carry out the design of prisms having excellent wide-angle properties as dictated by another object of the present invention.
In the aforementioned U.S. Pat. No. 1,591,572 to Stimson, there is described a method for making a prism mold by using glass rods or sheets having one end formed into the shape of a prism or prisms. However, the flat sheets used in the method described therein has such low strength that this method is not suitable for the formation of microprisms desired in thin retroreflective sheeting as dictated by an object of the present invention.
It is described in the aforementioned U.S. Pat. No. 3,069,721 to Arni et al. that optically flat metal faces can be obtained by cutting flat metal sheets with a diamond cutter and that prism sheeting can be formed by using prism-forming metal sheets obtained by this method. However, neither description nor suggestion is given therein as to the improvement of microprism characteristics by combining two or more different types of roof-shaped projections.
It is an object of the present invention to provide a master mold which can be applied to the production of cube-corner type retroreflectors, especially in the form of thin and flexible retroreflective sheeting, and permits the formation of microprisms having high brightness properties and excellent wide-angle properties, by focusing attention on the above-described plate method and overcoming its disadvantages while maintaining its advantages.